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HEPA filters ans laminar flow diffusers pressure drop

Hi Guys:

I'm not really too much in commercial duct design. I most of the time work in refrigeration racks, so forgive my ignorance. Here is the case: as part of the remodeling of a commercial fruit salad preparation / packing area, one of my clients wants to set up an air conditioning system provided with state of the art filtration and air flow. He wishes to install pre-filters, HEPA filters and laminar flow diffusers. All of these combined with the existing 5-ton commercial AHU, 3/4 HP blower motor. When I add up all the expected static pressure drops, I come out with a total external static pressure drop far greater than the unit's allowance, taken from the catalog. What can be done in this case? Some fellow tech recommended an in-line tube-axial belt driven booster fan, located downstream from the AHU, upstream from the pre-filters and HEPA filter racks. Is this the only option or someone knows about any other ? Thank You.

I doubt if a tube axial will give you the required pressure. A tubular centrifugal certainly would. Also it would have a much flatter fan curve as the filter loads up.
I would consider going away from the standard packed unit and going with something designed for this increased static as I never like the idea of two fans in series. (generally one of them ends up runnig backwards) Get a high pressure unit, they are available, and it would save allot of mickey mouse time playing around with controls and wiring.

If you have forward curved wheel in the existing packaged unit (which is very common) and if the "booster fan" starts first (or overpowers the wheel)it will start that wheel spinning in the opposite direction. If the speed is great enough there will not be enough starting torque in the motor winding to stop and reverse that rotation and the motor will run in the opposite direction. It will probably overheat, kick out on overload (about a thousand times before the overload melts) and eventually burn the motor windings to a melted blob of copper. If the motor does have enough torque the belts/pulleys may slip and everthing will certainly wear prematurely on both fans. The building will usually take on a very unpleasant odor in the process.
Trust me, i've been there, replace the unit or upgrade the fan in the unit. Adding a booster fan may look easy, and in some instances it is (usually low pressure) but it can get ugly real quick.

I am almost sure that with that small commercial unit there is no way to use Hepa Filter. And the fan booster it just a "patch". Remember that Hepa will increase the pressure loss in time.
I believe you don't need diffuser for, the Hepa usually does, you need a grille cover.